1. Field of the Invention
The present invention relates to a semiconductor laser element in which a light exit end surface is coated with a reflectance control layer which has high thermal conductivity. The present invention also relates to a process for producing such a semiconductor laser element.
2. Description of the Related Art
In the conventional semiconductor laser devices, the rise in temperature of a resonator surface during a high output power operation is considered to be a limiting factor of the maximum optical output, since a current which does not contribute to light emission is generated due to recombination at a surface of semiconductor layers corresponding to the resonator surface.
In order to solve the above problem, coating of the resonator surface with a group III nitride having high thermal conductivity is proposed. However, researchers belonging to the present assignee have found that the thermal conductivity seriously decreases due to oxygen capture by the group III nitride, and therefore the coating of the group III nitride does not function as desired.
The following patent publications disclose examples of coatings of the resonator surfaces.
(i) Japanese Unexamined Patent Publication No. 3(1991)-142892 discloses that reliability of semiconductor laser devices increases when resonator surfaces of the semiconductor laser devices are directly coated with AlN which has high thermal conductivity. However, the oxygen content in the AlN coating is not limited. Therefore, the high thermal conductivity may not be maintained during actual use of the semiconductor laser devices.
(ii) Japanese Unexamined Patent Publication No. 9(1997)-162496 discloses formation of nitrides of Al, Si, Ta, V, Sb, Mn, or Cr coating having a thickness of 0.5 to 10 nm on resonator surfaces. However, no disclosure is provided for oxygen concentration in the nitrides.
(iii) U.S. Pat. No. 5,063,173 discloses formation of Si3N4 coating on Si coating. However, since the thermal conductivity of Si3N4 is inherently low (i.e., 40 W/mxc2x7K or lower), characteristics of semiconductor laser devices are not substantially affected by the oxygen capture when the resonator surfaces are coated with Si3N4.
(iv) U.S. Pat. No. 4,656,638 discloses formation of a metal layer having a thickness of 10 nm or less on an end surface of a semiconductor laser element, and formation of a reflectance control layer, made of an oxide such as Al2O3, on the metal layer, where the metal layer is made of, for example, Al, Si, Ta, V, Sb, Mn, Cr, or Ti. However, the thermal conductivity of Al2O3 is 11 to 25 W/mxc2x7K, which is one order lower than the thermal conductivity of the nitrides.
As mentioned above, conventionally, the reflectance control layers made of group III nitrides are formed on the light exit end surfaces of semiconductor laser elements for the purpose of enhancement of heat dissipation. However, the characteristics and reliability of the conventional reflectance control layers are not sufficient. In addition, control of oxygen concentrations of the group III nitrides is not performed.
An object of the present invention is to provide a semiconductor laser element having improved characteristics and reliability, where the semiconductor laser element has a light exit end surface coated with a reflectance control layer having high thermal conductivity.
Another object of the present invention is to provide a process for producing the semiconductor laser element having improved characteristics and reliability, where the semiconductor laser element has a light exit end surface coated with a reflectance control layer having high thermal conductivity.
According to the first aspect of the present invention, there is provided a semiconductor laser element comprising: a multilayered structure being formed of a plurality of semiconductor layers made of a plurality of group III-V compounds, and having at least a pair of cleaved end surfaces as a pair of light resonator surfaces; a first reflectance control layer formed on at least one of the pair of light resonator surfaces, and made of an oxygen gettering material which has a function of gettering oxygen; and a second reflectance control layer formed on the first reflectance control layer, and made of a nitride of a group III material.
Preferably, the first reflectance control layer has such a thickness that the second reflectance control layer has an oxygen concentration of 5 atomic percent (atm %) or below, where the xe2x80x9catomic percent (atm %)xe2x80x9d is a percentage of atoms of an element of interest. Further preferably, the first reflectance control layer has such a thickness that the second reflectance control layer has an oxygen concentration of 3 atm % or below.
Since at least one of the pair of light resonator surfaces is coated with an oxygen gettering material which has a function of gettering oxygen, the oxygen gettering material in the first reflectance control layer can getter oxygen which is captured in the nitride of the group III material forming the second reflectance control layer. Therefore, it is possible to prevent the decrease in the thermal conductivity of the nitride of the group III material forming the second reflectance control layer, and the decrease in the maximum optical output due to heat generation at the light exit end surface during high output power operation of the semiconductor laser element. Thus, reliability of the semiconductor laser element can be increased. Further, when the second reflectance control layer has an oxygen concentration of 5 atm % or below (further preferably, 3 atm % or below), the thermal conductivity of the second reflectance control layer can be maintained high.
Furthermore, the semiconductor laser element according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (vii).
(i) The oxygen gettering material may be one or a mixture of group IV elements, one or a mixture of group V elements, or one or a mixture of other metallic elements.
(ii) The group IV elements may be Si and Ge.
(iii) The group V elements may be Sb and Bi.
(iv) In the case of (ii) or (iii), the thickness of the first reflectance control layer may be within a range of 0.2 nm and 50 nm.
(v) The other metallic elements may be Al, Ti, V, Ta, Cr, and Mn.
(vi) In the case of (v), the thickness of the first reflectance control layer may be within a range of 0.2 nm and 5 nm.
(vii) The group III material may be one or more of B, Al, In, and Ga.
According to the second aspect of the present invention, there is provided a process for producing a semiconductor laser element including a multilayered structure being formed of a plurality of semiconductor layers made of a plurality of group III-V compounds, and having a pair of cleaved end surfaces as a pair of light resonator surfaces, the process comprising the steps of: (a) forming a first reflectance control layer on at least one of the pair of light resonator surfaces, where the first reflectance control layer is made of an oxygen gettering material which has a function of gettering oxygen; and (b) forming a second reflectance control layer on at least one of the pair of light resonator surfaces, where the second reflectance control layer is made of a nitride of a group III material.
Preferably, the process according to the second aspect of the present invention may also have one or any possible combination of the following additional features (viii) and (ix).
(viii) The second reflectance control layer may be formed before the first reflectance control layer is oxidized.
(ix) The first and second reflectance control layers may be formed in a sputtering system which includes at least two evaporation sources, or in an evaporation system which includes at least two evaporation sources, or in a chemical vapor deposition system which is capable of forming at least two films.
When the first and second reflectance control layers are formed in a vacuum chamber, the first reflectance control layer is not oxidized. Therefore, it is possible to prevent the decrease in the thermal conductivity of the second reflectance control layer, and increase the maximum optical output power of the semiconductor laser element.